#2798 gatorwalk-factory: retarget a work item to another issue, journaled

Part of the tracker-seam decision (https://claude.ai/artifact/TDidgyVjxKXU7JsNH1baNy). Needs #2793.

Duplicates must move in-flight work to the primary issue, which means pointing a running work item at a different ticket. Today only start writes externalRefs (work_item_ops.ts:870 → run_ops.ts:143); nothing else does, including reset.

Change

• A work-item method retarget: inputs externalRefs, reason, and the usual expected stage, cycle and era.
• It replaces externalRefs and journals {type: "retargeted", from, to, reason, actor}.
• No gate and no stage change. Refused on a terminal work item.
• It is an engine method and knows nothing about trackers or duplicates. That is what keeps it on the engine side of the seam.
• publish uses the event to route: events before retargeted belong to the old ticket, later ones to the new. A cursor for the old ticket is kept, not refused.

Done when

• A walk test: retarget mid-lifecycle; gates, products and cycles are unaffected; the journal shows the event; the summary report shows both refs.
• Against the Lab fake: publish after retarget writes new events to the new ticket only.
• DESIGN.md "Tracker ids are data" says externalRefs changes only through start and retarget.